An Independent Evaluation of the El Hierro Wind & Pumped Hydro System

1. Overview

In the 1980s, the first ideas about the combination of a wind park with a pumped hydro system were discussed within Unelco, the then electric utility of El Hierro (“The Iron One”), followed by first principal designs in the 1990s. When support from the European Union was signalled, the company “Gorona del Viento” was founded in 2004.
In 2007, a cooperation agreement with the Instituto para la Diversificatión y Ahorro de la Energía (Institute for Diversification and Energy Saving) was signed for the further detailed planning and the joint implementation of the project. Also in 2007, the central and local government’s support for both wind park and hydro power plant was obtained. In the following year, international tender procedures started, building permit and environmental clearance were obtained. Civil works started in 2009, and the first equipment arrived at El Hierro in 2011 – two years later, the construction works could be completed, but the commissioning did not happen right away. Obviously, there were problems with the grid integration of both wind and hydro generators.

On June 27, 2014 the project was officially inaugurated, this being the start of a 12 month running-in period. Exactly 12 months later, in June 2015 the standard operation of the project started. In August 2015, the El Hierro electric energy system was, for the first time, operated by renewable energies alone – the thermal power station could be switched off for a period of 2 hours. In the first year of operation five further “diesel-off” tests were carried out, the longest of which lasted 55 hours. However, the originally intended prolonged operation without diesel power could not be effected. As such, and also officially since the beginning of 2016, the El Hierro Wind/Hydro Hybrid System is in what is called a “test operation”.

2. Project Goals

Originally, a 100 % renewable energy (RE) system for El Hierro, based on wind and pumped hydro power was targeted – and also “marketed” accordingly. The first design strategy aimed at 80 – 90 % of the wind energy used for water lifting, and the remaining to be fed directly to the electric grid. Considering energy losses for pumping, as well as the water turbines’ efficiency, it was found that the overall losses amounted to 40 % with such an approach. Thus, the targets were reversed and only about 20 % of wind energy was to aimed to go to the grid via the hydro power plant, the remaining 80 % should be fed to the grid directly from the wind park.

In order to allow the diesel generators to be switched off completely, the electrical generators of the Pelton turbines were designed in a way to function as rotating phase shifters(also called synchronous compensators or condensers). As a result, these generators were to supply the required short-circuit power and reactive power for the grid and to take over voltage and frequency control, when the diesel generators were disconnected completely (= diesel-off mode of operation).

After determining the final system parameters – in particular the size of the upper and lower water reservoirs which depended on detailed geological surveys – several system simulations based on measured wind and actual load data were undertaken – they predicted a typical annual RE penetration rates of between 64 and 68 %(2).

3. System Design

Starting point for the system design was the required annual electricity need of the island. The primary design parameter was the annual energy demand on El Hierro of 35 GWh, giving a calculated average load of 4 MW (minimum load of 3 MW, peak demand of 7.5 MW, load factor of 0.4). Assuming an initial annual demand increase of 8 % (later to reduce itself to 4 %) the calculations required a generation of 50 GWh from wind, meaning that 15 GWh or 30 % of the annual energy yield of the wind park was to cover the losses in the hydro system as well as the anticipated load increase.

Calculating the energy output for five wind turbines with 2.3 MW installed power each (Enercon E 70), the excellent wind regime of the intended wind park site resulted in an annual production of 49.6 GWh (wind park capacity factor of 49 %). The sizes of the hydro reservoirs could not be selected so freely – they depended on the geological conditions close to the pre-selected wind park site. The final design was an upper reservoir with 380,000 m3 and a lower reservoir with only 150,000 m3. The upper reservoir is constructed in an old, inactive volcanic crater, while the lower was excavated in a narrow valley close to the sea. When detailed geologic studies found weak rock under the planned reservoirs (and a similarly located water reservoir – Barlovento on La Palma – developed large leakages and failed in 2011) the size had to be reduced(3). The pumped hydro and wind project developed out of plans to have a large reservoir for irrigation in the inactive crater – this explains the size mismatch of the upper and lower reservoir. In order to use a full upper reservoir for electricity generation, 230.000 m3 of it would have to be dumped in the sea. But such an obvious waste of highly valued sweet water on a dry island – just to generate the islands electricity demand for 2 – 3 extra days – must be ruled out completely. Currently, no water is being used from the upper reservoir – so the extra size remains a complete waste …

Anyway, for the functioning of the pumped hydro only the 150,000 m3 of the lower reservoir is the decisive figure: together with the working head (the height difference between lower and upper reservoir) of 650 m the size of the lower reservoir determines the maximum storage capacity of 270 MWh(4). Then, with an average load of between 4 and 5 MW, such a storage capacity gives just between 54 and 68 hours of continuous electricity supply from hydro storage. Had the original intended size be realized (400,000 m3), 150 hours energy from storage would have been possible, about 6 days of average demand – and not just 3 – 4 days as the current storage.

4. Wind Resources and Storage Size

El Hierro disposes of excellent wind resources. Unfortunately, nowhere in the published documents the results of the original measurements at hub height have been published. But the indication of the “official” energy yield from the 5 Enercon E70 turbines (most likely provided for by the manufacturer itself) allows to re-engineer the annual average wind speed. Being within the influence of the trade winds on a hill-top location, one might estimate a Weibull wind speed distribution of k = 2.6 and an annual average wind speed of 9.5 m/s at hub height(5). This assumption results in a “realistic”(6) capacity factor of 49 % with the power curve of the E70 turbine and gives an annual production of 49.6 GWh. This high annual average wind speed is very much in line with measurements undertaken by the author in Cape Verde (Sao Vicente 9.8 m/s), Azores (Terceira 10.2 m/s) and on mountain sites at Morocco (Koudia El Baida – 10.6 m/s).

Referring to publicly available wind speed data from the El Hierro Airport(7) it has been suggested that an annual wind speed of 9.5 m/s at the wind park site was not possible, given the wind speeds indicated at the airport in only 3 km distance in the range between 6 and 7 m/s. The airport, however, is practically on sea level, while the wind park is located at a nicely rounded hill at around 650 m above sea level. As such, an increase of 50 % in the average wind speeds is highly likely, due to the hill-top effect (Figure 1). In addition, the wind speeds from the airport come from 5 – 10 m above ground, whereas the wind park features a wind measuring mast with a measuring height of 60 m (Figure 2). Therefore, an annual average wind speed of 9.5 m/s and a capacity factor of 49 % for the wind park seem plausible.

However, this excellent annual average wind speed comes with quite a considerable seasonal variation – in particular with two succeeding low-wind speed months in spring-time and autumn (typically in April-May and September-October). As a rule, for designing an autonomous energy supply system based on wind energy, the size of the storage is determined by the lowest wind production period. If one were to bridge-over two months with wind speeds of 6 – 7 m/s, the size of the storage (60 days * 120 MWh/day = 7,200 MWh) becomes absolutely prohibitive – it would mean reservoirs approx. 25 times as large as those built. Therefore, a careful analysis of long-term wind data needs to be undertaken, resulting in an optimization process between storage size and remaining diesel operation times. This has been done many times before – the results are easily predictable: while it is possible to arrive at a 100 % RE system, the storage needs are normally prohibitive – one has to design the system for the longest calm period in 20 years. So, in real life, one would settle with a RE penetration of about 80 %, the more so, since a diesel generator is needed anyway as a backup. With this in mind, and judging from similar good wind regimes from other islands, in particular Azores and Cap Verde, one can estimate that such an optimization process would have resulted in a storage size of between 10 and 25 days (= 1.200 to 3,000 MWh – i.e. more than 10 times larger than the existing storage.

Naturally, such a “storage optimization process” would have to be carried out as one of the first steps in project planning, before starting to do anything else. Knowing that 3,000 MWh needs two water reservoirs of 1,700,000 m3 it would have been clear from the beginning, that the topography of El Hierro (or at least that within close distance to harbour, existing diesel power plant and load centre, the main town of the island, Valverde) prohibits the construction of the required water reservoirs. In particular, such a simple “back of the envelope” calculation of storage requirements would have shown that a project with an energy storage of 270 MWh cannot be labelled a 100 % renewable energy project.

So why the responsible planners of the project marketed El Hierro as a 100 % RE systems remains a riddle. This goal cannot be achieved. Also the prediction of the two independent studies from 2010, which predicted between 64 and 68 % RE penetration must be regarded as flawed. Labelling them as “too optimistic” would be a euphemism.

5. Running-in Period and Test Operation

After a running-in and commissioning period of nearly 18 months, both wind park and pumped hydro plant started officially a test operation on June 27, 2015, aiming at a gradual increase of the RE penetration rate without endangering grid stability and security of supply. After 7 months, the diesel power plant was switched off for the first time for a period of 21 hours on January 31, 2016. In Figure 3 the hourly generation of January and February 2016 is shown: yellow – diesel, blue – wind or hydro, green – excess wind power used for pumping.

Figure 3: Generation in MW – January and February 2016

One can see that the RE-only periods on January 31 and February 20 were stopped when the wind dropped below the current grid load, whereas the tests on February 14 and February 29 had enough wind to continue. We can only assume that grid stability issues prohibited a continuation of the diesel-off operation. Also note the two black-outs of the complete system (“grid crash”) an February 18 and 19.

Unfortunately, neither Red Eléctrica de España (REE), from which generation data are available(8), nor GdV publishes wind speeds. But one can see clearly from Figure 3 that there was a period of 10 consecutive days from January 7 to January 17 with practically no wind at all. Such periods happen also in other months – see for example October 16 to October 26, 2015 in Figure 6. So, our earlier estimate that a storage to cover 10 days of calms (i.e. a minimum of 1.200 MWh) would have “saved” the El Hierro system from failure. When working with RE, experience tells that you always need a bit of spare capacity – thus, with 1,500 MWh (meaning a reservoir with 850.000 m3) El Hierro could have reached easily a 75 to 85 % RE penetration rate, provided all system components would have functioned as intended.

Intermediate summary: Had the storage capacity been designed according to our very primitive design rule from above, this period would have been bridged over. Another important fact is that the wind park did very rarely generate at nominal power (11.5 MW) – in the contrary, long periods of time the wind park output was deliberately curtailed, such as on January, 30 to 8 MW. There can be only one reason for this down-rating: grid stability problems. If the upper reservoir was filled completely, then the excess wind power for pumping (green) would have stopped. But the most surprising feature of Figure 3 is the fact that during the diesel-off tests excess wind power was used for water pumping. Normally, one would assume that the test starts with a full upper reservoir, and one limits the wind power to follow the load in the grid, knowing that the stored hydro power is working as a backup when the wind suddenly drops. So, our working hypothesis, it seems, namely that grid stability is the reason for the short periods of diesel-off operation, is confirmed: obviously, the water pumps (6 induction motors with 500 kW each and two variable speed pumps with power factor compensation) are needed for grid stability. In the very moment when wind and hydro power alone are supplying grid electricity (and the water pumps are not there to act as a controllable dump load and power factor compensation units), stability problems occur and the diesel generators are started again.
As such, we can conclude that the innovative feature of this project – pumped hydro storage – has not been employed so far to its full extent in diesel-off mode. A little bit cynical, we might add, therefore the absolutely wrong dimensioning of the storage could not have negative effects on the energy system.

6. First Year Results

On 30th June 2016, El Hierro completed the first full year of test operation. The results, as shown in Figure 4, are disappointing, considering the target wind energy penetration rate of ~ 65 %. Little more than half of it – only 34 % – was actually reached. The figures for pumped and lost wind energy through curtailing are estimations from Roger Andrews – he assumed a 60 % efficiency for the pumped hydro cycle. However, the lost production, i.e. the amount of wind energy not generated because the wind park output was down-rated, seems much too low, as the capacity factor of the wind park (originally calculated at 49 %) is now only 24.4 %. Such a capacity factor would be plausible at a typical German wind park site with annual wind speeds between 6 and 6.5 m/s – but not here in El Hierro with 9.5 m/s. Therefore a much larger ratio than 30 % must be assumed to be lost through curtailed wind energy through the rotor’s pitch regulation system.

Figure 4: First Year Results in MWh (Lost: curtailed wind power, Pump: used for water pumping)

This hypothesis is supported by the 10-min averages of wind generation for June 2016 in Figure 5: one sees that the wind park production was limited permanently to 7 MW, during the last 2 days of the months even to only 5.5 MW(9). This means that 2 of the 5 wind turbines are practically not used – idle investment which does not pay off. Unfortunately, when looking at the figures from hydro power, we find an even worse situation: only 3.9 % of the annual energy demand comes from the pumped storage – the installation for which 2/3 of the 82 million EUR was spent. We remember: originally planned was 20 % of annual energy from pumped hydro.

Figure 5: June 2016 – Wind Park Production as 10 min averages in MW

How rarely (and how inefficiently) the pumped hydro system was used becomes evident when looking at a graph which shows the actual production from the hydro power plant (Figure 6). The data for June 2016 (the month with the best wind resources, estimated to be between 10 and 11 m/s) reveal that on June 7 a test was made with hydro and diesel only (wind park was switched off) for about 14 hours, using about 27 MWh from the total of 270 MWh. The other hydro power employments were much shorter. Obviously, the hydro power tests on June 7 and 8 completely drained the upper storage, as the next 5 days (120 h) wind energy was used permanently with a load of ~ 3 MW to fill the reservoir (assuming 80 % efficiency for pumping, the reservoir was full again after June 14, when the next diesel-off test was started.

One might argue that during strong wind periods the hydro system is not needed as the wind alone is enough. But a look at the low wind season shows similar results (Figure 7): apart from the really short periods of time hydro is connected to the grid, it is the power range which ultimately documents the complete failure of the pumped hydro system: typically only 1 to 2 MW of hydro power is employed, whereas 11.3 MW is installed, of which 9.2 MW is available for grid feed-in. When comparing the October 2015 with the June 2016 data, one sees the striking difference: no curtailing of wind power at all in October, whereas in June 2016 the maximum wind power output is practically all the time limited to 7 MW. And, most of the time, the diesels are limited to 1.6 or 2.7 MW, while the Farm Control Unit (FCU), also delivered by the wind turbine manufacturer Enercon, managed the load following in a way that the access wind power is used for pumping.

The hydro power contribution over the year was 3.9 % of total energy, and the annual generation of 1.799 MWh results in a capacity factor of 2.2 %. Such a plant factor must be regarded as absolutely disappointing and the ultimate proof for a flaw in the overall design of the system, considering the costs, the environmental impact and the efforts which went into the pumped hydro plant(10). And that this pumped hydro scheme had been a unique selling point for the El Hierro system right from the beginning, makes things even worse.

7. Operation Modes

During test operation in 2015 and 2016 different modes of operation had been experimented with. GdV did not reveal the primary goal of these experiments – but we assume here that they wanted to optimize grid stability rather than renewable energy penetration rates. The functioning of the Farm Control Unit (FCU) can be seen clearly in the 10 min averages for March 9, 2016. Wind park output was limited to 5.4 MW, diesel output is fixed at 1.6 MW – and the FCU automatically splits the wind power between the grid and water pumping (Figure 8)

Figure 8: 10 min Averages from March 9, 2016

On this day we had excellent wind conditions, but the wind turbines were curtailed to just 5.4 MW – in effect, they supplied constant power output like a diesel generator. The diesel generator was set to 1.6 MW – this value we already noticed in previous graphs. While we do not know the operational strategy of GvD, we know that 4 of the 7 diesel generators of the thermal station have an identical rated power of 1.56 MW. As they operate on heavy fuel, their long-term minimum load must be assumed to be ~ 50 %: thus we get two 1.56 MW units operating in parallel at their minimum permissible load resulting in the observed steady output of 1.6 MW (as seen in Figure 8 and Figure 6, for example). By doing so, the system features a spinning reserve of ~ 1.6 MW should a sudden loss of load occur(11). This must be the reason we see the diesel for such long periods of time operating at 1.6 MW.

On May 16, 2016, GdV tested a completely different operation mode: here, under conditions of relative low to medium wind speeds (wind park output ~ 2 – 4 MW), the park control feature of the E70 was activated and limited the wind park output to 1 MW. Less than 1 MW was coming from hydro power, meaning that one of the four 2.83 MW Pelton turbines was running under 30 % load. The variations of the hydro power output, as visible in Figure 9, are unusual and cause the system to operate inefficiently, as the diesel generators do not only have to cope with load fluctuations from grid demand, but also with the variable output from the hydro power plant. While the FCU has been employed up to 07.00 h, it was switched off later, and only the Park Control feature of the wind turbines was used to keep total wind park output to 1 MW.

Figure 9: 10 min Averages from May 16, 2016

When comparing the 10-minute averages from hydro and wind one might draw the conclusion that the output regulation of the hydro system has some problems, while the wind turbines deliver a steady output practically as stable as a diesel generator. Maybe this is a hint to problems with the governor of the water turbines, and an explanation why the hydro power plant has only been used with an annual capacity factor of 2.2 %. It might also be the clue to the fact that in nearly 70 % of wind park operation time water pumps have been used: the FCU was functioning flawlessly, guaranteeing grid stability, while the hydro power plant did not.

8. The mysterious Water Issue

The original idea to install a wind park on El Hierro was the large amount of electricity needed for water lifting, mainly for irrigation (~ 40 % of total electricity went into water lifting). As such, not getting any information on the exact amount of water pumped and water used from the upper reservoir is a major disadvantage for a reasonable over-all evaluation of the system. What we know from the data REE makes available is that ¾ of the time the wind park generates electricity water is pumped into the upper reservoir. Since the upper reservoir has not been connected to the island’s piped water system(12), no water at all has been supplied for irrigation or drinking purposes.

Some evidence has been produced that water is pumped (and additional load is generated), but that this water does not reach the upper reservoir. Instead it is sent down-hill again into the lower reservoir through the valve house unused (see Figure 10)(13). It has been suggested that GdV uses the easily controllable water pumps (6 x 500 kW plus 2 variable speed 1.5 MW electric pumps with power factor compensation) to maintain sufficient load in the grid and control grid stability (mainly through the 2 variable speed pumps with a combined rating of 3 MW).

Figure 10: Water arriving at the lower reservoir without being used for electricity generation. Photo credits Rainer Strassburger

While there is strong evidence pointing in this direction, such an operation mode (and such an incredible waste of renewable energy with the associated wear and tear in the pumps and pipelines etc.) does not seem to make sense under consideration of the advanced grid interface of the Enercon E70 turbines and the obviously successful operation of the FCU. Being equipped with a direct drive, multi-pole synchronous generator, the E70 allows a power factor variation in a wide range. With an installed wind power of 11.5 MW this range should be more than enough to stabilize the El Hierro grid with a peak demand of 7.6 MW even under conditions of high wind energy penetration. Figure 11 shows the range of reactive power variation which is standard to all Enercon turbines(14).

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Considering the advanced grid interface of the Enercon turbines, the water “going-round-and- round” phenomenon most likely has another explanation, one only known to GdV. Maybe it is the strict Spanish grid code, to be said of not being different at the Canary islands than on the main-land, and the eagerness of REE to fulfil it 100 %(16). This, and a certain lack of courage (after some unexpected brown-outs or black-outs had happened) might have influenced REE to play it safe(17). But power factor problems, or more generally, grid stability problems from wind park operation must be considered as unlikely, as the Enercon turbines are one of the most “grid friendly” and versatile turbines on the market and as the generators of the Pelton turbines (11 MW) are designed as rotating phase shifters. So before anyone would pump water uphill by means of variable speed electric motor pumps with power factor compensation, a considerate system operator would use the ability of the wind turbines and/or the synchronous condensers of the Pelton turbines to improve grid stability.

9. Conclusion from One Year of Test Operation

Optimistic observers of the El Hierro pumped hydro and wind system had hoped to see a gradual increase of the RE penetration rate over time and a more regular operation in diesel-off mode, given the facts that

the energy system has been designed from the beginning to allow diesel-off mode (generators of Pelton turbines designed as synchronous condensers – rotating phase shifters);

a hydro power plant able to cover more than the absolute peak load in the grid (currently 7.6 MW – hydro power: 9.2 MW), and

a wind park with 11.5 MW installed power with the (calculated) ability to completely supply the island’s current annual electricity demand of 46 TWh.

However, an improvement of the RE penetration rate over time did not happen. With 34.6 % wind and hydro content in the electric grid in the first year of operation the performance of the El Hierro systems has just reached about one half of what was predicted. Given an expected wind production of 49 TWh the 24 TWh actually generated in the first year of operation are less than half of the calculated potential. Whether the wind park experienced a typical wind year or not and whether the lower generation has to be attributed to a beyond-average wind year, cannot be said. The regular down-rating of the turbines to typically 7 MW (= 60 % of rated output), however, is a clear indication of large amounts of generation loss due to sub-optimal operation of the wind park.

The storage capacity of the pumped hydro plant is completely wrongly dimensioned – a fact a simple spreadsheet calculation based on one year of measured wind data (and without doing any complicated simulation runs) would have shown right from the beginning. That the project planning and detailed design continued with such an obviously insufficient energy storage capacity (by the factor of 10 to 20) can only be attributed to a combination of political influences and wishful thinking, possibly also to a strongly voiced ambition of the inhabitants of El Hierro to get cheaper (or even free) irrigation water and the pleasant feelings of the project planners to engage on a 100 % RE project. Technical explanations for the grossly wrong dimensioning of the storage system cannot be identified. By people who know the socio-political situation at El Hierro and, more generally, at the Canary Islands very well, these planning mistakes are attributed also to a general attitude of local corruption; they imply that the project planners knew from the beginning that what they promised to the subsidy givers was technically not possible(18). Some even describe the political situation on El Hierro as being Byzantine or “Erdogan-like”.(19)

In addition, the usage pattern of the first two years of operation indicate that (a) either a constant lack of water supply is impeding the operation of the pumped hydro, and/or (b) a lack of trust in the geological stability of the upper reservoir keeps the operators from filling it completely(20).

Given the situation that not all technical details are public knowledge (in particular not the nature of the obvious grid stability problems when the diesel power station contributes less than 1.6 MW), the following proposals are made:

With a contribution of just 3.9 % of annual energy demand and a maximum power of ~ 2 MW, the hydro plant is useless – the production in the first year did not even pay the annual running costs. Therefore it is proposed to completely shut it down;

Instead, operate the wind park in high-penetration or ultra-high penetration grid-parallel mode, through varying the values of curtailment in a load-follow fashion, relying on the wind turbines for reactive power variation.

To better understand these proposals, a digression into the principal modes of wind/ diesel operation is necessary.

Operation modes of wind turbines and diesel generators

In principle, four modes of operation exists for wind and diesel hybrid systems:

Mode 1 – standard grid-parallel operation (Figure 12): here, up to 50 % of the absolute minimum load in the grid are installed as wind power, thus making sure that the diesel power always has the lead. This operation mode is the least technically demanding, as the maximum wind power, at any time, can be integrated into the grid. The disadvantage, however, is the limited achievable wind penetration rate: Assuming a minimum load of 4 MW for El Hierro, a single 2.3 MW turbine could operate in standard grid-parallel operation and would generate at 49 % capacity factor about 10 GWh/a. The result would be a wind energy penetration rate of 22 % (assuming an annual energy demand of 46 GWh):

Figure 12: Standard grid-parallel operation – schematic (Mode 1)

Mode 2 – high-penetration grid-parallel operation (Figure 13): using the same simple grid stability rule as for standard grid parallel operation – maximum wind contribution > 50 % of actual grid load – the high-penetration grid-parallel operation still does not require any additional installations in the electric infrastructure of the isolated grid (i.e. no power factor compensation devices etc.(21)). As such, this operation mode results in the highest penetration rates with the least effort. The dimensioning is equally simple as in mode 1, only this time we reference the maximum wind power installation to the system’s peak load. For El Hierro with ~ 8 MW this mode allows the installation of 2 wind turbines with 2.3 MW each. Theoretically, a 50 % wind penetration rate is possible, if the wind turbines would operate with 100 % capacity factor. Assuming that 20 % of the available wind power has to be curtailed (typically at night time) we arrive at 2 x 10 GW minus 20 % = a net wind generation of 16 GWh/a, giving us a wind energy penetration rate of 35 %.

Mode 3 – ultra high-penetration grid-parallel operation (Figure 14): an absolute minimum load for the diesel power station is determined, which normally is maintained by means of special measures (such as the employment of low-load diesel generators(22)). Such a long-term extreme low-load operation is normally only possible with diesel generators running on light fuel (minimum long-term low-load operation ~ 25 %). Given the fact that in El Hierro heavy fuel diesel generators are operated (minimum load ~ 50 %), this mode will not result in any substantially higher penetration rates than mode 2(23).

Mode 4 – wind/diesel systems, allowing diesel-off mode. This is by far the technically most demanding mode, requiring high technical efforts, but promising the highest wind energy penetration rates, in practice ranging from 40 % to 80 %, depending on the system layout. It is clear, that originally the El Hierro project was designed to operate as a wind/diesel system, i.e. most of the time in diesel-off mode. To understand the technical challenges of wind/ diesel systems, Figure 16 shows their main components. To allow diesel-off operation, a Master Synchronous Machine MSM is needed (= a rotating phase shifter or synchronous condenser – in El Hierro the electric motors of the Pelton turbines), a long-term energy storage (here a battery – in El Hierro the pumped hydro) and a short-term energy storage, a flywheel (see Figure 15) to level out the short-term variations from wind (unfortunately, no equivalence in El Hierro). To use the hydro power generators to level out power fluctuations from wind in a similar way a flywheel would do is technically not possible as the reaction time of the Pelton turbines (inertia) is far too large when compared to a flywheel.

Figure 15: How a flywheel levels out short-term fluctuations coming from wind

While such a flywheel for the levelling-out of short-term fluctuations works fine, it has its obvious limits for larger power demands(24). With the technical proress achieved in the meantime in battery manufacturing, today the employment of batteries for output levelling (i.e. as power stabilizer, see Figure 17) are the better option.

Apart from acting as a power stabilizer, batteries in these wind/diesel hybrid systems have another important function: due to their short reaction time, they act as a replacement for spinning reserve, meaning they monitor the grid permanently and supply their rated power should they detect a sudden loss of load. In practice the battery capacity is sufficient for only a few minutes – but this is long enough to start another diesel generator in the power station, synchronize it and bring it online. Then the battery is automatically disconnected and reverses into charging mode. This function is extremely important for high-penetration wind systems, as a sudden loss of wind energy in a situation with insufficient spinning reserve would cause an immediate black-out of the complete energy supply system. In order to avoid such a situation without battery storage the diesel power station needs to operate constantly with enough spinning reserve to cover the complete load. This is a very inefficient mode of operation as several diesel generators have to operate at minimum load in parallel for extended periods of time(25).

Figure 18 demonstrates the effect of a battery system working as a “spinning reserve” substitute: here, on the high-penetration system of the island of Bonaire, a sudden loss of load is experienced, when, within 13 seconds, 5.7 MW of wind energy dropped to zero. The battery kicks in (3 MW rated power, 3 MWh storage capacity) and bridges the time until another diesel (in this case also heavy fuel like at El Hierro) can be started and brought on line. A similar system as in Bonaire is installed at the Wind Park Baltra/Santa Cruz in Galápagos. Here two kinds of batteries are employed: lead acid batteries for peak shifting and for providing a “virtual” spinning reserve and Lithium-Ion batteries for load-levelling (Figure 17). In order to cover the losses of the battery system (charging losses, inverter losses etc.) the Galápagos system in Figure 19 has been equipped with a PV-system.

These two examples of functioning high-penetration grid-parallel projects(26) were given in more detail above, as one must assume that the lack of short-term energy storage – and consequently, the lack of both power stabilizing and spinning reserve function is in the centre of the current problems of the El Hierro system. The operator GdV (or REE being responsible for keeping grid stability) , when experimenting with diesel-off mode (see Figure 3 and 6), probably was in permanent fear that a sudden loss of wind power combined with a delay of hydro power cut-in would cause a black-out. It seems most likely that this was the reason why the originally planned standard operation mode (“diesel-off”) was only tested for a few hours at a time (now a maximum of 55 hours in a row)(27). No doubt that these tests were not successful – or at least GdV and REE did not regard them as successful – otherwise they had had plenty of opportunities to operate in diesel-off mode, given the high wind resources of El Hierro.

As a working hypothesis, it is assumed that the responsibility for the implementation of the Enercon Farm Control System (FCS) was limited to the automatic start of the water pumps(28), and did not include the equally automatic start of the Pelton turbines. Maybe they tried to include the hydro turbines in the FCS but failed to succeed, owing to a too long reaction time of the Peltons. This seems unlikely, as the hydro power plant in El Hierro is supposed to use a novel “no-flow operation of the turbine-driven generators”, meaning that, once the electrical generator of the Pelton turbine is successfully synchronized with the grid, the water jet is closed and the generator is operating as no-load motor, consuming about 2 % of rated power (~ 50 kW)(29). When the grid frequency goes down, the jets are opened and the water turbine can take over load immediately, as the Pelton wheel is already synchronized. Thus, this special “no-flow” operation of the Peltons overcomes a major disadvantage of all hydro power plants, i.e. the time they need to reach synchronous speed from standstill(30). Without “no-flow” operation a Pelton turbine would not be in a position to compensate a sudden loss of load, when operating in parallel with wind power plants. Thus, running the Peltons on El Hierro in “no-flow” operation is a technical pre-requisite for the automatic parallel operation of hydro and wind and for disconnecting the diesels. In addition, the synchronous generators of the Peltons supply short-circuit power, deliver reactive power and take over the voltage regulation even under no-load conditions.

Inertia constants for hydro power are in the range of 1 to 3 s(31), while wind turbines feature a typical inertia constant in the range between 2 and 5 s(32). This is because their rotor acts like a large flywheel. The Enercon E 70, being a direct drive turbine without any fast-running parts (maximum speed of the generator/rotor: 21 rpm) is estimated to have an inertia constant in the range of 5 to 7 s, owing to the high weight of the multi-pole, large-diameter generator and the kinetic energy stored in the generator/ rotor assembly. This high inertia is similar to (large) conventional power plants, and is the reason why Enercon is in a position to offer an option for their turbines, called inertia emulation. When this optional feature is installed, the turbines can support “conventional power plants in stabilizing the grid frequency or even take over this task completely.”(33) Even when operating at nominal power of 2.35 MW, the E70 is able to supply an additional 250 kW to avoid the grid frequency to fall out of its preset range (see Figure 20) – simply by taking out more kinetic power stored in the generator/rotor assembly.

So, if GdV indeed experienced grid stability problems when running hydro without diesel power, they should have asked Enercon to add inertia emulation to their turbines – this would have stabilized their grid perfectly, the more so, as the turbines are generally operated down-rated to ~ 7 MW.

That the FCU did not include the hydro generators, therefore, might not be attributed to technical reasons. More likely are contractual limitations: while Enercon was responsible with their FCU for wind park and pumping management, the supplier of the hydro power equipment was in charge of the hydro turbine control. In the end, a full system integration was not reached, and the manual control was so complicated (and risky, with the view to grid stability) that GvD decided to not operate in “diesel off” mode with hydro power anymore.(34)

Whatever the reason, the drastic under-usage rates of the hydro system demonstrates clearly that it is technically not in a position to reliably replace spinning reserve in the diesel power system and therefore forces the power plant operators to always operate with a minimum of two diesels in parallel, both operating on their absolute minimum load. In contrast, the high-penetration systems on Bonaire and Galápagos can operate their diesels according to the actual residual load and trust that the battery system kicks in in case of problems or a sudden reduction of wind speeds.

Also the fact that GdV obviously never experimented with a situation when the hydro power was supplying the complete grid load (or in combination with wind) seems to be an indication for the lack of power stabilizing in the grid, i.e. the lack of short-term energy storage. A standard hydro power plant is often not in a position to level out short-term load fluctuations coming from wind power(35). This is the reason why the wind/diesel system on the island of Flores (Azores) introduced a flywheel storage (500 kW Piller, controls by PowerCorp, Australia) into their wind/hydro/diesel system. Only then it was possible to operate their system reliably on hydro and wind, and switch off the diesel completely.(36)
In conclusion, there would be two possible approaches for the current problems – make the hydro power plant work as intended (which, as a minimum. would need the introduction of additional short-term storage – batteries or flywheels) or disregard the hydro completely, concentrate on high-penetration grid-parallel operation of the wind park and move on gradually to a full-fledged wind/diesel operation – at least during the strong-wind period, where enough energy is available.

In case the water storage would be in a position to bridge over realistically major low-wind speed periods (for this a capacity rather of 1.500 to 3.000 MWh instead of 270 MWh would be needed) the first option would be the recommended one. But this solution is technically not possible. Therefore, only the second option remains: For this an additional investment for a 3 – 5 MW battery system with 3 – 5 MWh storage capacity would be needed: it would increase the virtual spinning reserve and take out power fluctuations and allow the reliable operation of the wind park in diesel-off mode for extended periods of time.

In high penetration grid-parallel operation a wind energy system with just 2 x 2.3 MW wind installation would give us a RE penetration rate of ~ 35 %. This is exactly the same RE penetration El Hierro has achieved during the first year of (test) operation. But then, to get this 35 % penetration rate, a generation of 25 GWh from wind was needed, of which 3 GWh was used for pumping and 7 GWh was curtailed. If one considers the maintenance needs for 5 wind turbines (instead for 2) and all the additional efforts for maintaining the reservoirs, penstock, pumps and hydro turbines operational, the inefficiency of the El Hierro project becomes clearly visible.

Finally: Once the hydro power component is disregarded, and a concentration on the operation as a wind/diesel system has been made, i.e. a strategy is implemented which maximizes “diesel-off” mode, El Hierro has the potential to reach its original project target (60 – 80 % wind penetration). To achieve this, additional investments for a battery system are needed – for power stabilization and – more importantly – for the introduction of virtual spinning reserve. As a side effect, such a battery system can be employed for peak shifting, which increases the economic operation of the thermal power plant.

It is ironic to propose another expensive investment for an already extremely expensive and uneconomic energy system. But this additional investment is small when compared with the investment for the pumped hydro power scheme which will never reach its payback. The tragic of the current situation, that practically half of the investment for the wind park does not contribute to the return of the project and that the hydro power part is only used with 2 % capacity factor, can only be remedied through the move to a full-fledged wind/diesel system with long-time operation in “diesel-off” mode. Hopefully this decision will soon be taken.

So why the responsible planners of the project marketed El Hierro as a 100 % RE systems remains a riddle.

Because it sounds good, no riddle at all.

Antius on Sat, 25th Feb 2017 12:23 pm

Hydro is a poor energy storage medium. 1m3 of water lifted to 2000m contains 20MJ (5.6kWh) of potential energy. That makes pumped storage a capital intensive means of energy storage. And you need right topography for it to work at all. Not much good in the UK, no good at all in Holland.

Wouldn’t it be really cool, if we had a magical energy source that was as clean as wind power, was completely controllable and had economics as good as coal power without the emissions? I bet no one in their right mind would build a wind turbine if they had an energy source like that.

GregT on Sat, 25th Feb 2017 12:30 pm

“Because it sounds good”

Just like “renewable” sounds good. People only believe what they want to hear.

The people who are upset with you for speaking the truth, are those people who are living a lie.

All of which are completely dependant on fossil fuels. Take fossil fuels out of the equation, and none of it gets built.

GregT on Sat, 25th Feb 2017 1:47 pm

“Hydro is a poor energy storage medium.”

Our electricity is 100% from hydro, and has been for the better part of a century.

Cloggie on Sat, 25th Feb 2017 1:59 pm

Artius talks about storage, you about generation.
Get an engineering education GregT.
One Heinberg is bad enough.

GregT on Sat, 25th Feb 2017 2:03 pm

All of our reservoirs are energy storage mediums, until the water is used to generate electricity.

Cloggie on Sat, 25th Feb 2017 2:29 pm

Artius was talking about pumping water UPWARDS.

GregT on Sat, 25th Feb 2017 2:35 pm

“Artius was talking about pumping water UPWARDS.”

We use solar energy for that. No need for fossil fuels.

GregT on Sat, 25th Feb 2017 2:49 pm

Artius was also talking about mining asteroids with robots, and space colonization.

We have far more pressing issues to deal with, other than the continuation of any semblance of modern industrialism or BAU.

Better to focus on food production, water security, and Climatic instability.

Cloggie on Sat, 25th Feb 2017 3:43 pm

Agree.

Antius on Sat, 25th Feb 2017 4:03 pm

‘Artius was also talking about mining asteroids with robots, and space colonization.
We have far more pressing issues to deal with, other than the continuation of any semblance of modern industrialism or BAU.
Better to focus on food production, water security, and Climatic instability.’

GregT, intermittency has three different components and we need a combination of storage technologies to continuously match supply and demand on the grid. Hydroelectric dams are an energy efficient means of storing power, but energy density is poor. What that means in practice is that hydro is useful for smoothing out short term supply-demand inconsistencies on a scale of hours. Used in that way, pumped storage is efficient and economical. But wind, wave, solar and tidal experience strong seasonal fluctuations. They happen to be our most abundant renewable energy sources. If we are to rely on these for power, European countries will need to store TWh of energy over periods of many months. That is a very different task to daily grid balancing. The storage medium must be cheap and energy dense. Stored biomass and stored heat can do this. Stored heat is less energy efficient than hydro, but because huge quantities must be stored over long periods, the lower capital costs make it a much more cost effective option.

The third component is inter-annual fluctuation. This is the most difficult of all and is probably best dealt with using stored chemical fuels.

The problem with mass expansion of renewables to cover all energy requirements is the sheer scale of European energy demand. The best renewable sources in terms of EROI are onshore wind, hydro, biomass, tide and potentially wave. With the exception of wave, these are environmentally disruptive. They are all limited in ultimate realistic supply. We could use all of them and still need more. Solar PV and offshore wind are larger resources but have poorer EROI. Sensibly, we would only prioritize these projects only after investment opportunities in better EROI options become scarce. Come the next financial crisis, there will be some very tough decisions to make.

This isn’t the right place to talk about space colonisation. But suffice to say, the problems we are facing here on Earth will never stop getting worse because we are depleting a closed system. Solving one problem always creates more. Solving the energy problem would be disastrous to the biosphere. Growing more food cannot avoid impacting ecosystems. Using extra resources to build infrastructure reduces the amount available to future generations. It doesn’t matter ultimately how much priority you decide to attach to problems, you will not solve them in a closed system. As I have said before, in a closed system, every success begins to look like a failure.

GregT on Sat, 25th Feb 2017 6:52 pm

Thanks Antius,

Completely agree. Predicaments do not have solutions. Only difficult choices, and the biggest predicament we face is overpopulation.

Tick tock.

peakyeast on Sat, 25th Feb 2017 7:23 pm

@Antius: Yeah.. I concur. Good comment Antius. Thanks.

Davy on Sat, 25th Feb 2017 7:44 pm

Antius, great to have you on board to assist with alternative energy technicals.

green_achers on Sun, 26th Feb 2017 7:27 pm

“It doesn’t matter ultimately how much priority you decide to attach to problems, you will not solve them in a closed system. As I have said before, in a closed system, every success begins to look like a failure.”

First, it’s not a closed system. It receives, and has received as long as life has existed on the planet, virtually all of it’s energy from the sun.

Second, “success” is quite simple, and what humans did for their entire existence before they unlocked all of that stored solar carbon. And what every other life form does eternally: Live within the real-time solar budget, augmented mostly by what can be stored in biomass.

makati1 on Sun, 26th Feb 2017 8:45 pm

green, but those same humans also burned it so fast the planet couldn’t get rid of the heat energy and so … there is no future for humans after. The planet will go on until the sun goes nova. There will be life evolve from whatever survives homo sapiens, but it will not be humans. Maybe two meter roaches or rats the size of elephants? We will never know. Our time has come and gone. (I won’t mention the hundreds of thousands of tons of nuclear wastes and all of the billions of tons of chemical poisons we have dumped into the biosphere in the process. Ooops!)